1. The Field of the Invention
The invention relates to tires intended to bear heavy loads, such as, for example, aircraft tires.
2. The Related Art
Aircraft tires must withstand extreme conditions during service, in particular in terms of applied load and speed, taking into account their low weight and size. As a result, despite their very high inflation pressures, greater than 12 bar, their loading or deflection during operation may commonly reach values double those observed for heavy-vehicle tires or passenger-car tires.
During takeoff, very high speeds, of the order of 350 km/hour, are achieved, and hence heating conditions exist which are also very severe.
All these conditions are particularly disadvantageous for the endurance of the beads of these tires.
U.S. Pat. No. 4,832,102 describes an aircraft tire comprising a crown, two sidewalls and two beads, a carcass reinforcement and a crown reinforcement, in which the carcass reinforcement comprises two circumferential alignments of reinforcing threads of high elasticity modulus anchored in the two beads. The crown reinforcement comprises at least one working block with at least one ply of reinforcing threads of high elasticity modulus. The carcass reinforcement is anchored in the beads by turning up, around a bead wire, the two circumferential alignments of first reinforcing threads of high elasticity modulus.
Patent documents EP 0 582 196, EP 0 664 232 and U.S. Pat. No. 5,660,656 have recently proposed, in the case of passenger-vehicle tires, a novel type of carcass reinforcement for a tire and of the anchoring thereof in the two tire beads. A tire, in accordance with these applications, comprises a bead with means for anchoring the carcass reinforcement comprising reinforcing threads oriented circumferentially and axially bordering the circumferential alignments of the reinforcing threads of the carcass reinforcement.
Application WO 98/54006 (U.S. Pat. No. 6,109,321) proposes a tire, the carcass reinforcement of which comprises two or three circumferential alignments of reinforcing threads in which each circumferential alignment is bordered axially to the inside and to the outside by reinforcing threads oriented circumferentially. This application mentions numerous types of reinforcing threads for constituting the circumferential reinforcing threads.
In what follows, “linear density” is understood to mean the weight in grams of one thousand meters of a reinforcing thread. The linear density is expressed in tex. The stress to which a reinforcing thread is subjected and the modulus of this reinforcing thread are expressed in “cN/tex”, cN meaning centinewton.
“Reinforcing thread” is understood to mean any reinforcement element in the form of a thread which is able to reinforce a given matrix, for example a rubber matrix. As reinforcing threads, mention will be made, for example, of multifilament yams, these yams possibly being twisted on themselves or not, unit threads such as single cords of high elementary diameter, with or without a twist on themselves, cabled yams or plied yarns (“cords”) obtained by cabling or plying operations on such unit threads or yams, such reinforcing threads possibly being hybrid ones, that is to say, composite ones, comprising elements of different natures.
“Plied yarn” (or “folded yam”) is understood to mean a reinforcing thread formed of two single yarns or more assembled together by plying operations. Such single yams, which are generally formed of multifilament yams, are first of all plied individually in one direction (S or Z direction of twist) during a first plying stage, then twisted together in the opposite direction (Z or S direction of twist, respectively) during a second plying stage.
“Adherent reinforcing thread” is understood to mean a reinforcing thread which has undergone an appropriate coating treatment, referred to as sizing or adherization treatment, capable of making the reinforcing thread, after suitable heat treatment, adhere to the matrix for which it is intended.
“Axial” is understood to mean a direction parallel to the axis A of the tire. This direction may be “axially inner” when it is directed towards the inside of the tire and “axially outer” when it is directed towards the outside of the tire.
“Radial” is understood to mean a direction perpendicular to the axis A of the tire and passing through the axis A. This direction may be “radially inner” or “radially outer” depending on whether it is directed towards the axis A or towards the outside of the tire.
“Substantially circumferential orientation” is understood to mean an orientation which does not diverge by more than five degrees from the circumferential direction.
“Elasticity modulus” of a rubber mix is understood to mean a secant modulus of extension at 10% deformation and at ambient temperature.